The comprehensive objective of this project is to better understand thalamic function, and more specifically, to define the role of the ventral posterolateral nucleus (VPL) of the thalamus in tactile integration, using a multidisciplinary approach that correlates single neuron recordings and selective cortical inactivation in the alert primate, with thalamocortical connectivity. Processing of complex spatiotemporal patterns applied to the hand is of paramount importance in manipulation and tool usage, as demonstrated by sequelae of VPL lesions, and is even more crucial to the blind. Consequently, it is important to define the functional roles of neuron populations at each level of the somatosensory system, in order to understand how tactile information is processed. While the receptors, primary efferent fibers, and primary somatosensory cortex (SI) have been explored in some detail, intermediate nuclei such as the VPL have received less attention. The proposed study will, for the first time, classify VPL neuronal responses to conventional, moving, and OPTACON stimuli in the alert primate and reveal the fine grain physiological organization of this nucleus, with particular emphasis on the hand representation. The OPTACON is a visual substitution device providing precise control of parameters in a complex patterned stimulus. It has previously been used to demonstrate transformation in the coding of spatiotemporal information that occur between the periphery and cortex. This study will also explore the function of cortical feedback onto thalamic sensory neurons and by selective reversible inactivation of corticothalamic neurons reveal the intrinsic properties of VPL neurons. Finally, the connective between the fine grain topographic sub-modality organization in VPL and physiologically defined subregions of cortex will be ascertained using retrograde tracers. In summary, increased knowledge of VPL structure and function will contribute to our comprehension of mechanisms underlying tactile integration and the role of the thalamus vis-a-vis cortex. It may also provide the basis for better understanding of sensory loss due to thalamic lesions and for generating more sophisticated tactile substitution devices for use by individuals with visual and auditory handicaps. GRANT=R29NS28012 Imidazoleacetic acid (IAA), which we demonstrated in rat brain and rat and human CSF by gas chromatography-mass spectroscopy, stimulates GABA-A receptors and has numerous other pharmacological and biochemical effects in the brain. We also discovered in brain, micromolal levels of conjugated IAA (probably the ribotide and/or riboside), the levels of which in brain and CSF exceed those of free IAA up to 100-fold, which may be indicative of IAA turnover. Although IAA and its conjugates are known to be present in peripheral tissues, the heterogeneous distributions in the brain and the concentration gradients for both IAA and its conjugate(s) in CSF suggest origins in brain. The regional and subcellular distributions of endogenous IAA, IAA-conjugate(s) and of IAA's putative metabolizing enzyme, imidazoleacetic acid phosphoribosyltransferase (IPRT), will be measured in aliquots of rat brains after perfusion to remove blood which contains IAA. 3H-conjugates (unavailable commercially) will be prepared and used as standards. After rats are injected intracerebroventricularly (i.c.v.) with 3H-IAA (synthesized in this lab), its 3H-conjugate(s) and any other metabolites in brain, CSF and plasma will be separated, isolated and identified by mass spectrometry. The effects of probenecid on levels of endogenous IAA and its conjugate(s) will be examined. Turnover of endogenous IAA in brain will be approximated by measuring the rate of increase of IAA after giving salicylate (i.c.v.), an inhibitor of IPRT. 3H-IAA will be given (i.c.v.) without salicylate and the rate of formation of conjugate(s) will be measured. The origin of IAA will be assesses. As preliminary data suggest that IAA in brain may be derived physiologically from histidine with imidazolepyruvate as an intermediate, brain IAA will be measured after histidine loading in rats in which histidine decarboxylase is irreversibly blocked with alpha-fluoromethylhistidine (alpha-FMHis). Formation of IAA will also be measured in brain homogenates preincubated with salicylate and alpha-FMHis, and incubated with increasing concentrations of histidine. Since at high concentrations, histamine in brain may be enzymatically oxidized to IAA, homogenates preincubated with salicylate will be incubated with histamine and inhibitors of diamine of monoamine oxidases. To estimate the contribution of peripheral IAA and its conjugates to brain IAA and its conjugate(s), low concentrations of 3H-IAA will be infused for several days with a minipump implanted in the peritoneal cavity, and the specific activities of 3H-IAA and 3H-conjugates in plasma and brain will be measured. This work will reveal the origin, metabolism, disposition and turnover of IAA in brain, and serve as a prelude to evaluate its function in health and disease and its role in the action of neuropsychopharmacologic agents.